There has been considerable recent interest in the development of new ambient cure coating systems. A major consideration in this regard is the desire to replace existing systems which require elevated cure temperatures, systems which contain or release toxic substances during cure, and systems which are generally slow to cure.
For example, in the "color-plus-clear" coating systems for automotive topcoats, final coats of a clear coating are applied over a pigmented basecoat. Known color-plus-clear coating systems based on thermosetting resins require cure temperatures of at least 120.degree. C. In addition, one- and two-part polyurethane systems rely on organic isocyanates for their cure, and therefore suffer from the physiological hazards associated with the inhalation of, or skin contact with, these compounds. In an effort to overcome these disadvantages, current technology has focused on the use of systems which rely on alkoxysilane functionality for a less toxic, more rapid ambient cure mechanism.
U.S. Pat. No. 4,499,150 to Dowbenko et al. discloses a color-plus-clear coating method wherein a vinylic addition copolymer containing alkoxysilane and/or acyloxysilane functionality is present in either the basecoat or the topcoat. However, the curing time of such a system may still be slow, even when a curing catalyst, such as dibutyltin dilaurate, dibutyltin dimaleate or tetrabutyl titanate is added. A prolonged cure time after application of the coating to the substrate can result in cracks or other undesirable properties.
The above application of a vinylic copolymer which contains hydrolyzable silane moieties in its molecule is but one illustration of the utility of such systems. Of these copolymers, those having oximosilane-functional groups can offer particular advantages as ambient cure, one-part coatings: they exhibit faster, more thorough cure and they show improved physical and chemical properties over the corresponding alkoxysilane-functional copolymer coatings.
However, as desirable as these oximosilane-functional vinylic copolymers may be, their heretofore disclosed methods of preparation are somewhat limited and have been observed to be relatively inefficient relative to our discovered preparative method, described infra. Thus, to date, essentially three procedures have been describe for the preparation of vinylic copolymers containing various types of hydrolyzable silane functionality. For example, Plueddemann, in U.S. Pat. No. 3,453,230, teaches room temperature curable acrylate rubbers wherein a mixture of an acrylic monomer, a silane monomer which contains an unsaturated group as well as a hydrolyzable group, a mercapto-functional silane and a free radical catalyst is polymerized in the absence of water. The resulting copolymer can be cured by exposure to moist air.
In U.S. Pat. No. 4,157,321 to Kawakami et al., copolymers of an ethylenically unsaturated organic monomer and an unsaturated organosilane monomer are shown to be stabilized by the addition of a compound selected from monomeric hydrolytically reactive organosilanes or trialkyl orthoformates. The copolymers of this invention are prepared by conventional addition polymerization methods.
Furukawa et al., in U.S. Pat. No. 4,578,417, disclose a moisture curable composition having improved storage stability comprising a hydrolyzable silyl group-containing vinyl polymer and an orthoacetic acid ester as a stabilizer. The polymers of this contribution to the art are said to be prepared by either the above mentioned addition polymerization technique or by hydrosilation of a vinyl polymer having a carbon-carbon double bond with a hydrosilane.
U.S. Pat. No. 4,795,783 to Hunt teaches coating compositions which comprise a blend of a hydroxyl-functional vinyl polymer and an organopolysiloxane containing hydrolyzable groups. The latter, in turn, is prepared by the partial hydrolysis of a silane containing hydrolyzable groups.
Chang et al. teach acrylic-silane copolymer compositions in U.S. Pat. Nos. 4,043,953, 4,684,697 and 4,714,738. Again, these disclosures do not suggest the method of the present invention, but do discuss three conventional ways to prepare the copolymers. One of these methods is the above mentioned procedure of addition polymerization of an acrylic monomer together with an alkoxysilane having acrylic functionality thereon in the presence of a free radial initiator. A second method involves the hydrosilation of an acrylic addition copolymer containing carbon-carbon double bonds with a hydrosilane in the presence of a transition metal catalyst. The third method contemplates a reaction between a hydroxyl-functional acrylic resin with a minimal quantity of certain organosilicon-containing materials, such as organosilicates or their partial hydrolysis products.